One type of material commonly employed in fabricating ohmic contacts is metal silicides such as cobalt silicide. Cobalt silicide and other metal silicides are typically fabricated using a conventional self-aligned silicide (salicide) process, wherein a blanket TiN/Co film is deposited over the devices and annealed to form cobalt monosilicide over the exposed silicon regions (source, drain and gate) of transistors in USLI integration. A selective wet etch is employed to remove the TiN cap and the non-reacted cobalt left over the oxide or nitride regions. The cobalt monosilicide is then subjected to a second anneal which converts the monosilicide into a cobalt disilicide layer. The cobalt disilicide phase has a lower resistance than the cobalt monosilicide phase.
The above self-aligned silicide process cannot be performed using a single anneal because of the diffusion of silicon (Si) atoms in the cobalt (Co) film along the sidewalls of the transistor. If the first anneal is at too high a temperature, Si can diffuse in the cobalt over the oxide/nitride regions of the device and will not be removed by the etch so that the source and drain areas become shorted to the gate. This phenomenon is known in the art as bridging. The self-aligned silicide process relies on the selective wet etch for removing any left over metal alloy or metal from the exposed nitride/silicon regions. As the device dimensions are further reduced, the constraints on the Si diffusion will become more stringent.
Moreover, and as one skilled in the art is aware, the silicide formation consumes a considerable amount of silicon. The thickness of Si consumption for a Co disilicide film is 3.6 times that of the initial cobalt film. As the junction depth of active regions becomes shallower, this large Si consumption becomes a problem.
In view of the above drawbacks with prior art salicide processes, there is a continued need for developing a new and improved method that is capable of limiting silicon consumption and reducing bridging during metal silicide formation.